Leishmania donovani promotes macrophages polarization towards M2 phenotype in vitro: A new approach to identify a new therapeutic target

Background: The immune response against L. donovani depends significantly on infected macrophages.
Since Leishmania amastigotes deploy several immune suppressive mechanisms to escape host immune
responses, macrophages polarize towards the classically activated macrophages (M1) or the alternatively
activated macrophages (M2). The balance between both types is crucial in shaping the infection outcome.
Objective: The aim of this study is to explore the macrophage polarization behavior in response to L. donovani
infection, and to examine the differential expression of IL-10 and TNF-α by each phenotype.
Material and Methods: Leishmania-infected phorbol 12-myristate 13-acetate (PMA)-treated human
leukemia monocytic cell line (THP-1) was used as an in vitro model of Leishmania infection. Leishmania
stationary phase promastigotes were used to infect the macrophages at different multiplicities of infections
(MOIs) i.e., ratio of macrophages to stationary phase promastigotes at 1:1, 1:10, and 1:20; and time points of
24 and 48 h post infection (PI). While CD68, CD40, HLA-DR were used as markers for M1; CD68 and CD163
were used to characterize M2. Both M1 and M2 phenotypes were analyzed using flow cytometry. To evaluate
the behavior of polarization, IL-10 and TNF-α were tested in both phenotypes, in addition to the assessment
of percentage of infected macrophages.
Results: The percentage of M1 exhibited significant decrease followed by non-significant increase, while M2
showed significant increase correlating with the MOIs. Both phenotypes expressed MOI-dependent increase
in IL-10, but only M1 significantly expressed TNF-α. Besides, M2 phenotype predominated M1, in a time and
MOI dependent manner.
Conclusion: Leishmania infection induces macrophages polarization towards M2, with significant production
of IL-10. These results extend knowledge regarding the immunomodulation exerted by Leishmania
amastigotes to defeat the immune system.